The present invention relates to a light dimmer circuit which uses two silicon control rectifiers (SCRs) to alternately energize a load, which can comprise one or more lamps, at adjustable points in the respective half cycles of an input AC voltage sine wave.
It is known in the prior art that a triac can be used to energize a load such as a lamp during alternate half cycles of an input AC voltage sine wave, and that the firing angle of the triac can be adjusted by various means to achieve a desired amount of dimming of the lamp. However, a problem which arises with the use of a triac is that, especially for high power light dimmers on the order of 2,000 watts or more, large heat sinks are required to dissipate the heat and to prevent possibly dangerous temperatures being reached in the dimmer circuit.
In addition to the aforementioned problem of a single triac in a dimmer circuit acting as a single point source of heat, leading to a hot spot, there is also a requirement for a low voltage drop for such triacs. This requirement necessarily increases the cost of these devices. Also known is a control circuit for progressively varying the illumination intensity of lamps comprising a single silicon controlled rectifier (thyristor) connected in series with a lamp load. This single SCR is gated by a unijunction transistor together with a charging circuit having a charging time higher than the charging time of the gate biasing circuit of the SCR such that the current flowing through the lamp load is progressively and gradually decreased at each cycle of the alternating current till it reaches zero, and afterward is increased to its full intensity in a similar cyclical manner.
Also known is a dimmer circuit wherein two SCRs respectively operate independently on opposite halves of the alternating current sine wave to convey current to a load such as a lamp. However, since these dimmer switches each provide a half-wave output having a direct current component, load devices suitable for only alternating current operation may not be employed. Also, unlike the present invention, the dimmer switches are connected in series with the AC voltage source in line with only one of the two source conductors supplying the load. Also, in the circuit of the present invention, each SCR also provides a voltage with a direct current component. The two direct currents however are in opposite directions end so, if the circuit components are symmetrical, there should not be any net D.C. Furthermore, the inventive circuit is also in series with only one of the two source conductors. The load is between the circuit and the other source conductor.